This invention relates generally to television cathode ray picture tubes, and more specifically, to means for alleviating the build-up and retention of undesired localized high-potential static charges on the surface of the cathode ray tube envelope.
The standard circuits for supplying operating voltages to the components of a cathode ray tube and associated electron gun are shown in highly schematic form by FIG. 1. A cross-sectional view of a fragment 10 of a funnel of a cathode ray tube will be readily recognized by those skilled in the art. The section as shown comprises a glass envelope 12 with an anode button 14 passing therethrough and receiving a high voltage. Anode button 14 makes electrical contact with an inner conductive coating 16, usually comprised of colloidal graphite deposited on the inner surface of the funnel. An outer conductive coating 18 of similar material is also located on the funnel. Outer conductive coating 18, being grounded, does not contact anode button 14, but is kept away from anode button 14 by an electrically isolative gap 20 between outer conductive coating 18 and anode button 14.
Power supply 22 supplies potentials to the electrodes of electron gun 24; these potentials are in the range of a few volts for operation of the filaments (not shown) of electron gun 24 to as high as thirty-two kilovolts for operation of the accelerating anodes of gun 24. The relatively low potentials are shown as being supplied to electron gun 24 through leads 26. The aforesaid high potential is supplied through lead 28 to anode button 14, which is in electrical contact with inner conductive coating 16. The final accelerating anode of gun 24, anode 24A, receives the high potential conducted by inner conductive coating 16 through a plurality of "snubber" springs, which are not shown but which are represented schematically by connection 30. The outer conductive coating 18 is brought to electrical ground through lead 32 to complete the power supply circuit.
The glass 12 of the cathode ray tube funnel serves as the dielectric of a capacitor formed by the difference in electrical potential between outer conductive coating 18, which is shown as being at ground, or zero, potential, and the relatively high potential of inner conductive coating 16, which may be a potential of thirty-two kilovolts. The capacitor so formed serves as a component of the filter circuit of the power supply 22. This capacitor, in conjunction with other power supply components, acts to smooth out high-voltage peaks in the power supply circuit. The capacitor is characterized by a relatively large capacity--in the range of one to two thousand picofarads--a capacity due to the fact that inner conductive coating 16, and outer conductive coating 18 cover extensive areas of the inner and outer surfaces of the cathode ray tube funnel.
The large capacity of the capacitor so formed, and the high potentials involved in tube operation, can be the source of many problems common to the cathode ray picture tube art. These problems result mainly from the retention of the high potential charge by the aforedescribed capacitor, retention of the high potential charge on the anode button, and build up and retention of high-potential static charges on normally uncoated areas of the tube; that is, on bare glass areas.
It takes an appreciable length of time to completely discharge any capacitor. In a television cathode ray picture tube, the capacitive charge may persist for several seconds, hours, or even months after the television set has been turned off. One effect of this high potential charge retention may be a persistent glow visible on the television screen for several seconds after set turn-off. Further, charge retention of the capacitor can present a safety problem. When servicing a set, the service technician may not wait the considerable time it may take for the charge to disperse; as a consequence, the technician may experience an electrical shock. Under most conditions, the shock may constitute only an annoyance; however, under certain conditions (the presence of perspiration, physical contact with an electrical ground, etc.), the residual charge can present a definite hazard.
Even though the technician takes the precaution of momentarily shorting the anode button to ground, the shock condition can recur. The glass of the envelope which provides the dielectric becomes polarized, and may remain polarized even though the plates of the capacitor--the inner and outer conductive coatings--are deliberately discharged. After a period of time, the high potential charge will build up again due to the polarization of the glass and the condition contributing to shock will again be present. The high potential charge can be retained even after long periods of storage of the tube.
Any residual high potential charge remaining on the anode button after television set turn-off may result in a white dot appearing on the screen of the tube. Television circuits commonly include a bleeder resistor in the power supply to discharge the high potential on the anode button.
The build up and retention of localized high-voltage static charges on normally uncoated areas of the tube envelope can result in an annoying "crackling" sound which may be heard upon set turn-on and turn-off. This crackling sound is due to a "rush" of electrons which creates an audible microscopic sparking on the tube surface. The crackling sound has its origin in the electrical energizing and deenergizing of the inner conductive coating upon tube turn-on and turn-off. FIG. 1A illustrates another fragment of tube envelope 10 of FIG. 1. This is a section of the envelope wherein a bare glass area 15 is located opposite inner conductive coating 16, such as the area of the neck, or of the flange. When the external power supply is turned ON as indicated schematically by switch 13, inner conductive coating 16 will rise, or be "driven upward" to a potential of thirty kilovolts, for example. Bare glass area 15 instantly rises to about the same potential by capacitive coupling. The effect is indicated highly schematically by capacitor 17 and 19. Capacitor 17 represents the equivalent capacity of the relatively large effective capacitor formed by inner conductive coating 16, the dielectric supplied by glass 12 of the envelope, and the charge-collecting bare glass area 15, the surface of which acts in effect as a capacitor plate. Relatively small capacitor 19 represents an equivalent capacity formed in conjunction with the "environment" surrounding the tube envelope. These capacitive effects, plus a modicum of electrical leakage of the glass 12 of the tube envelope, results in the aforedescribed "rush" of electrons and the consequent crackling sound. If switch 13 remains in the "ON" position for a sufficient length of time, "discharged" portions of glass surface 15 may tend to revert to a potential close to ground potential. When switch 13 is turned to OFF and inner conductive coating 16 is driven to ground potential, the potential of previously "discharged" bare glass area 15 is capacitively "driven downward", charging its potential by essentially thirty kilovolts, and causing previously "discharged" areas to assume a potential of essentially 30 kilovolts below ground. The rush of electrons recurs, this time in the opposite direction, and the crackling sound is again heard.
Although primarily a "cosmetic acoustic" problem potentially annoying to television set users, the basic cause of the crackling sound may, under certain conditions, be the precursor of more serious arcing able to destroy components and circuits. The electron gun is highly susceptible to damage from such arcing, as are external electronic circuits such as the video drivers and circuits comprising transistors or integrated circuits. The magnitude of some arcing currents can also result in the actual puncturing and destruction of the envelope of the cathode ray tube itself.
Destructive arcs can be initiated by such localized high-potential static charges. So serious is the problem of arcing that elaborate measures are commonly taken to completely suppress arcing, or at least to alleviate its effects. Referring again to FIG. 1, an arc-suppressing impedance shown schematically by block 34, may be introduced into the circuit between inner conductive coating 16 and the accelerating anode 24A of gun 24 to suppress or otherwise ameliorate any arcing that may develop in the region of the electron gun. To protect external circuits, another arc-suppression circuit, indicated schematically by block 36, may be located outside the cathode ray tube envelope, comprising a part of the external electronic circuits. Typical circuits and components for suppressing arcing and/or reducing its effects include decoupling networks which act as buffers; also, spark gaps, diodes, filters, gas-discharge lamps and various resistive devices. These may be termed "contingency components" that may become useful only upon occurrence of an arc, and as such, contribute little or nothing to improving television set performance, while constituting a substantial cost factor.
The build up of localized high potential static charges in gap 20 between anode button 14 and outer conductive coating 18 may trigger an arc therebetween. The localized areas constitute discrete islands of high potential static charge which may link together to provide a path for arcing. A compound known as "corona dope" may be required to prevent such localized charge build up and consequent arcing. Corona dope is a spray which is applied to an area around an anode button, such as the area of gap 20 to prevent the build up of such charges. If the use of corona dope is necessary, a vinyl anode cap cannot be used because the material of the corona dope degrades the resistive properties of the vinyl. As a result, when corona dope is required, it becomes necessary to use a more expensive anode cap comprised of silicone, a material which is unaffected by the corona dope. Practical means to prevent arcing between the anode button and the outer conductive coating would result in cost savings in that of less costly vinyl anode cap can be used. Also, the cost of the material and the labor required for the application of corona dope could be eliminated.